Holochroal eye: found in nearly all Orders few to very many lenses (to >15,000!) lenses typically small, numerous one corneal layer covers all lenses lenses in direct contact with others no sclera between lenses corneal membrane covers surface only.
Schizochroal eye: found in some Phacopida only typically fewer lenses (to ca 700)
lenses much larger, fewer each lens bears an individual cornea lenses separated from each other sclera between lenses very deep corneal membrane extends into sclera.
Abathochroal eye: found in Cambrian Eodiscina only few lenses (to ca 70) lens size small, not numerous each lens bears an individual cornea lenses separated from each other interlensar sclera not deeper than lenses corneal membrane ends at lens margin.
All early trilobites (Cambrian), had holochroal eyes and it would seem hard to evolve the distinctive phacopid schizochroal eye from this form. The answer is thought to lie in ontogenetic (developmental) processes on an evolutionary time scale. Paedomorphosis is the retention of ancestral juvenile characteristics into adulthood in the descendent. Paedomorphosis can occur three ways: Progenesis (early sexual maturation in an otherwise juvenile body), Neoteny (reduced rate of morphological development), and Post-displacement (delayed growth of certain structures relative to others). The development of schizochroal eyes in phacopid trilobites is a good example of post-displacement paedomorphosis. The eyes of immature holochroal Cambrian trilobites were basically miniature schizochroal eyes. In Phacopida, these were retained, via delayed growth of these immature structures (post-displacement), into the adult form.
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